The only "bad" kind I can think of would be ones that were not waterproof. Like, burn-your-home-down heat mats suitable for sore muscles but not around water.
I always assumed that any sold commercially would have passed some safety examination and be certified lawyer-resistant.
I think the main quality issue would be "are you also going to buy a soil thermostat to control the temperature?" I think the thermostats cost lots more than the heat mats.
Lots of people do without a thermostat by knowing what temperature the room will be, and trusting the manufacturer's estimate that the mat will raise soil temp by 5-15 degree F ... or whatever they estimate.
But that TOTALLY depends on the thermal insulation under and around the heat mat. Imagine that you put a heat mat on top of some steel-wire-shelving in a cold room with a strong draft. It would provide hardly ANY heat to the tray! All the heat would blow away, and especially through evaporation from the soil surface if there was no humidity dome.
Now picture a heat mat on top of some sheet rock (gypsum board, an insulator). Assume there is a humidity dome over the tray, and the area is protected from drafts. In this "insulated" case, the tray might become 15-20 degrees warmer than it would have been without the heat mat.
i think in practice we just put a hat mat under some seeds that we think need it warmer than the grow room. If the seeds seem to do better, we keep using the heat mat. Or you can turn the heat mat sideways and heat HALF of each of two trays. If the seeds in the warm halves do better than in the cool halves, that variety of seed liked it warmer. (It helps to keep records from year to year, especially tracking how warm the room is and whether it has any drafts.
Assuming the mat itself IS waterproof, all it does is generate a certain amount of heat uniformly over its area.
Here are three commercial examples, for one, two or four flats each the size of a 1020 tray:
$23.50 — 9" x 19.5" (1 flat), 17 watt - - - - 175 sq in - - 9.7 watts per 100 sq in
$39.50 — 20" x 20" (2 flats), 45 watt - - - 400 sq in - - 11.25 watts per 100 sq in
$63.50 — 48" x 20" (4 flats) 107 watts - - 960 sq in - - 11.1 watts per 100 sq in
